Browsing by Author "Boshernitsan, Valeriya"
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- Electrical characterization of resistive switching devicesPublication . Boshernitsan, Valeriya; Gomes, Henrique Leonel; Mariano, Fernando Morais LopesThe aim of this thesis is to investigate the electrical properties of resistive switching memories. Two distinct device architectures were explored. One involves a sandwich type of structure based on lithium fluoride (LiF) thin film, and the other is based on the integration of cadmium sulfide (CdS) nanoparticles within various polymer matrices. The research work encompasses the fabrication and detailed morphological and electrical characterization of the fabricated devices. Electrical and optical measurements carried were complemented by a morphological characterization of CdS nanoparticles. A variety of electrical techniques, namely, current-voltage measurements, small-signal impedance, constant current stress, and electrical noise-based techniques were used to characterize the devices. The CdS nanoparticles were embedded in different insulating host matrices, including photographic gelatin, polyvinylpyrrolidone (PVP), polystyrene sulfonate (PSSNa) and dimethylformamide (DMF). The physical and chemical properties of CdS nanoparticles in those different matrices were investigated. CdS nanoparticles (NPs) have been characterized using several techniques such as XRD and SEM analysis, absorption, and photoluminescence (PL) emission spectroscopy. This characterization allowed us to establish the structure of the thin films as well as the role of the pH values, the type of host polymer matrix on the nanoparticle’s stabilization. Free-standing films prepared using a high concentration of nanoparticles in a PVP host matrix were also electrically characterized. Both types of device structures, the LiF diodes and the CdS based devices undergo an electroforming process that permanently modifies their electrical properties. In the case of LiF diodes, the electroforming creates defects that lead to a memory device with two well-defined resistive states. In the case of the CdS nanoparticles embedded in a PVP matrix, electroforming changes the behavior from a semiconducting type to an insulating behavior. The physical phenomena that lead to the permanently induced changes are discussed in detail. In the case of the CdS the electroforming is described as a Coulomb blockade that leads to the formation of a nanodielectric type of material. In the LiF-based diodes electroforming is described as due to the formation of electrical bistable defects on the LiF layer.